The present invention relates to a motor control apparatus for controlling the speed of a motor and, particularly, for setting the speed of the motor.
FIG. 6 is a circuit diagram of a conventional motor control apparatus disclosed in, for example, Japanese Patent Laid-Open No. 87689/1985, and FIG. 7 is a diagram showing a conventional circuit for setting the speed of a sewing machine disclosed in, for example, Japanese Patent Laid-Open No. 136093/ 1984.
In FIGS. 6 and 7, reference numeral 21 denotes a commercial power source, 22 denodes a converter for converting the commercial power source into DC electric power, and 23 denotes an inverter which inverts the DC electric power converted through the converter back to AC electric power of a desired frequency. Reference numeral 24 denotes an electric motor, 25 denotes an encoder for detecting the speed or position of the motor, and 26 denotes a waveform shaping unit for shaping the waveform of pulse signals from the encoder 25.
Reference numeral 27 denotes a speed detector circuit which detects the speed of the motor upon receiving signals from the waveform shaping unit 26, reference numeral 28 denotes a speed feedback signal, and 29 denotes a speed instruction signal that is set by and produced from a speed setting circuit of FIG. 7.
Reference numeral 30 denotes a motor control apparatus, 31 denotes a variable resistor for setting a high speed shown in FIG. 7, and reference numeral 32 denotes a variable resistor for setting a low speed shown in FIG. 7.
The conventional motor control apparatus is constituted as described above. The operation will now be described in conjunction with FIGS. 6 and 7.
Referring to FIG. 6, when the power source 21 is connected to the main circuit unit of the motor control apparatus 30, the electric power causes the motor 24 to rotate passing through the converter 22 and inverter 23 in the main circuit unit. The encoder 25 coupled to the motor 24 sends to the waveform shaping unit 26 the pulse signals of a number proportional to the rotational angle of the motor. The waveform shaping unit 26 which has received the pulse signals determines the forward rotation or the reverse rotation, shapes the waveform, and sends signals to the speed detector circuit 27. Upon receipt of the signals, the speed detector circuit 27 produces a speed feedback signal 28 for the motor 24.
The speed instruction signal 29 of FIG. 6 is obtained from the speed setting circuit of FIG. 7. By setting the variable resistor 31 or 32 of FIG. 7, the speed instruction signals 29 of a frequency corresponding to the speed are sent to the motor control apparatus 30. Upon receipt of speed instruction signals 29, the motor control apparatus 30 compares the speed instruction signals 29 with the speed feedback signals 28, and so controls the motor 24 that its number of revolutions becomes in agreement with the speed instruction signals 29.
FIG. 8 is a diagram which schematically illustrates a conventional motor control apparatus equipped with an A/D converter disclosed in, for example, Japanese Patent Laid-Open No. 97118/1982, Japanese Patent Laid-Open No. 206284/1982 and Japanese Patent Laid-Open No. 208881/1982, and FIG. 9 is a diagram of characteristics representing the speed of a motor corresponding to the voltage obtained through a variable resistor.
In FIG. 8, reference numeral 41 denotes a commercial power source, 42 denotes a motor, and 43 denotes an encoder for detecting the speed or position of the motor.
Reference numeral 44 denotes a speed feedback signal, 45 denotes a speed instruction signal, and 46 denotes a variable resistor for setting a high speed.
Reference numeral 47 denotes a variable resistor for setting a low speed, 48 denotes a motor control apparatus, and 49 denotes a first A/D converter which converts an analog quantity from the variable resistor 46 into a digital quantity.
Reference numeral 50 denotes a second A/D converter which converts an analog quantity from the variable resistor 47 into a digital quantity, and 51 denotes a switch.
The conventional motor control apparatus is constituted as described above. When the power source 41 is connected, the electric power causes the motor 42 to rotate passing through the main circuit unit in the motor control apparatus 48.
To set the speed of the motor 42, the speed instruction voltage obtained by the setting of the variable resistor 46 or 47 is applied to the first A/D converter 49 or the second A/D converter 50 in the motor control apparatus 48. The first A/D converter 49 or the second A/D converter 50 receives the speed instruction voltage and converts it into a digital quantity. The speed instruction voltage converted into a digital quantity is selected by a switch 51 and is produced as a speed instruction signal 45. The motor control apparatus 48 compares the speed instruction signal 45 with a speed feedback signal 44 from the encoder 43 that is coupled to the motor 42, and so controls the motor 42 that its number of revolutions comes into agreement with the speed instruction signal 45.
When the speed is to be set using the above-mentioned conventional motor control apparatus, the variable resistor must be manipulated while measuring the number of revolutions of the motor using a tachometer or the like. Therefore, the speed is set requiring extended periods of time and resulting in an increase in the cost.